Thursday, October 06, 2005

Off-Airway Navigation

When determining a route of flight, there is no real need to run on an airway. If two NAVAIDs are in close enough proximity to each other that at least one can be received at all times, filing NAVAID direct NAVAID is perfectly acceptable. You fly outbound on one until you can receive the other. This is the basic idea behind airways in the first place. There is a little more to it, though.

Airways are assessed for obstacle clearance and NAVAID coverage. Hence you'll see at least an MEA (Minimum Enroute Altitude). This guarantees signal coverage continuously through the route. If the airway segment is safe at a lower altitude, a MOCA (Minimum Obstacle Clearance Altitude) may also be published. Flying below the MEA but at or above the MOCA means that you're above terrain and obstructions, but you may be below signal coverage at least at some points between airway segments. With no airway, you don't have the benefit of these established altitudes to know you're safe. The basic rule of IFR flight now comes into play: Ensuring you're at least 1,000 feet above the highest terrain or obstruction within 5NM of the aircraft. Now, if you're in one of the 5 Designated Mountainous Areas (detailed in the AIP and the Designated Airspace Handbook), then you have to be 1,500 or 2,000 feet, as appropriate, above the terrain and obstructions. This can be tougher to figure out and be 100% sure of. If you're operating off airway, then the pilot is responsible to ensure his altitude is acceptable. One thing you can use is a GASA, or Geographical Area Safe Altitude found on LO charts, as well as, perhaps, overlapping 100 NM safe altitudes from airports where instrument approach plates are published. Just make sure that whatever altitudes you use are good for your route segment, and include a reasonable overlap just in case you drift a little off course.

One way pilots can file flight plans including route portions where NAVAIDs without airways between them are used is the radials/tracks they'd be flying between them. For example, the pref route published for FL180 and above out of Halifax for Toronto has "YHZ 303 MLT VLV" and onwards, with the 303 between YHZ and MLT representing the YHZ 303 Radial. The idea is exactly what was described above: Flying outbound on the YHZ303R until you receive MLT and flying direct MLT from that point on whatever radial you end up on when you pick it up. Now that's a high altitude pref route and obstacle clearance is not an issue at that altitude for the Moncton FIR.

If you do your homework right, you can find a safe altitude for the route segment to be flown. It would be wise, also, to become very familiar with the surroundings along that route, just in case you have a problem, such as encountering ice that you can't get above and end up having to descend instead. It would be nice to know a course of action that's possible to keep you out of harm's way with the terrain around your route if worse were to come to worst.